How big should I build my broody pen?

I am going to be building a broody pen on a 4X8 platform that comes off of my coop. The plat form is about 3 feet of the ground. I am going to build it like a mini coop so that it is preditor proof and will block drafts. How big will I need to make it so that my hen will be comfortable, and be able to raise her chicks in there?